Lyric discussion by alphabravo 

No one can understand the lyrics of this song without first watching the John Wayne/John Ford film, "The Searchers". That is the film that the line "that'll be the day" was originally drawn from. In the film, "that'll be the day" is the signature phrase of John Wayne's tired, world-weary gunman; not a phrase that's at all about bravado, but a phrase that denote bitterness and loss of hope and resignation. In the masterpiece of film-making that was "The Searchers", the phrase was a characteristic of an important character. Buddy Holly raised it to a cultural icon.

"That'll be the day", in the days of the Hawks Code, was an aphorism for 'fuck no, not going to happen'. The phrase originated in Old Western films, and is used here as it always was: a way to indicate defiance, even in an atmosphere where defiance might not have been acceptable.

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